I have a desktop with two 1920x1080 monitors. Actually my xorg.conf contains this configuration ( my main monitor DFP-0 is on the right side and my secondary monitor, detected as DFP-2, is on the left side. ) :

All is working fine as soon as I want a special single monitor metamode to run games. I tried the trick involving additional metamode with a NULL display ( "DFP-0: 1920x1080 +0+0, NULL" in the above configuration ).

When I switch to this secondary metamode ( with ctrl alt + ), my secondary monitor correctly shuts down, but I still get a very large 3840x1080 virtual screen on my main monitor, being able to scroll it with my mouse. Every window has the same position on the virtual screen as previously with dual monitor. I don't know how to fix to get a real single monitor resolution...